This invention relates to new ceramic compositions with useful properties extending into high temperature ranges of 800.degree. C. and above and more particularly to ceramic compositions containing in dispersed form an additive class which absorbs stress-related energies transmitted in the composition. These additives may be characterized as transformation-prone rare-earth niobates, tantalates or mixtures of these with each other and/or mixtures with rare-earth vanadates. The dispersant is incorporated as small particles in a ceramic matrix in order to attenuate or hinder the propagation in the matrix of cracks that arise from stress.
Ceramics have been proposed as replacements for metals in a number of important products including those for high temperature use such as turbine blades, cylinder liners and heads for internal combustion engines, connecting rods, and the like. Ceramics have also been used as liners in various industrial processing equipment including equipment for coal gasification.
Ceramics in general are composed of hard brittle materials such as Al.sub.2 O.sub.3, ZrO.sub.2, MgO and mixtures of oxides such as mullite (Al.sub.2 O.sub.3 and SiO.sub.2) As small particles, these materials are mixed with sintering aids and sintered at elevated temperatures to cause the particles to adhere together in the desired shape. Stresses in these materials caused by temperature variations and/or mechanical loads limit the performance of the ceramics. Excessive stresses cause cracks to form and grow across the structural shapes. In some instances, these cracks may travel at or near the speed of sound and are not effectively hindered nor blocked.
Some additives have been incorporated into ceramic compositions to reduce the effect of stresses on performance. One of these involves the addition of an alloying ingredient (CaO is typical) to ZrO.sub.2 in order to achieve a structure in which the matrix consisting of the cubic form of ZrO.sub.2 contains as a dispersant fine particles of the tetragonal form of ZrO.sub.2. Another involves adding ZrO.sub.2, wholly or partially in the tetragonal form, to Al.sub.2 O.sub.3 or mullite. The principal mechanism believed to be associated with the beneficial effect of both of these additives involves the action of an advancing crack in producing a stress-induced transformation of the tetragonal form of ZrO.sub.2 to the monoclinic form with a resultant volume change. A change in the free energy of the system occurs and, as a consequence, there is an attenuation of crack progression. Further details are provided in the article "Transformation Toughening by Dispersed Polycrystalline Zirconia" by R. Stevens and P. A. Evans, Br. Ceram. Trans. J., 83, 28-31 (1984).
While the effect of a tetragonal ZrO.sub.2 dispersant in a cubic ZrO.sub.2 matrix or in an Al.sub.2 O.sub.3 or mullite matrix has interesting results, this approach is limited in its upper operating temperature. This is because the tetragonal form of the ZrO.sub.2 dispersant necessary for the beneficial effect cannot be retained above about 800.degree. C. Moreover, it appears that the tetragonal ZrO.sub.2 dispersant functions only by the volume-change effect of the transformation.
Still another limitation in the use of tetragonal ZrO.sub.2 as a dispersant at lower temperature as well as higher temperature is the need for a complex, difficult-to-control interplay of additive composition thermal treatment and dispersant particle size that is difficult to manipulate in achieving satisfactory level of crack attenuation. When higher operating temperatures of 800.degree. C. to 1200.degree. C. are required for ceramics; and/or when additional crack-attenuating functions are sought and/or when better crack-attenuation control is required at any operating temperature: other dispersant additives would be desirable.
Accordingly, one object of the invention is an additive system to improve crack attenuation at higher temperatures in ceramics. Another object of the invention is an additive system which acts as an energy absorber through one and preferably more than one mechanism. An additional object of the invention is an additive system which provides a range of additives useful, effective and controllable under different temperature conditions and in different ceramics. A further object of the invention is an additive system combining two or more of the above objects. These and other objects of the invention will become apparent from the following detailed invention.